POWER October 2012 - 22

2. In search of the best baffle. Boilers with tight flow path
turns can have a significant amount of flow separation to the rear of the
unit, which causes serious FAE in areas highlighted by hashes. To reduce
separation and lower FAE, two sets of flow-modifying baffles are
shown that have significantly different results. The cascade arrangement
(C) redistributes fly ash and gas in stages, but it does not provide
an adequate solution for this type and size of boiler. In contrast, an
angled baffle (A) spans the unit width and prevents much of the rearward
flow separation and is the appropriate FAE baffle system for this
unit. Source: Drennan Engineering Inc.
Operational Results
Baffles (C in Figure 2) were installed in an earlier attempt to
address FAE. This installation is a cascade arrangement that attempts
to redistribute flow in stages. FAE was reduced at the rear
wall, but unfortunately at the cost of increased FAE in the center
of the bank. For smaller units (<400 MW), cascade arrangements
deflect too much flow into the center of the bank. In larger units
(>800 MW), there is more distance and time for deflected flows
to dissipate before entering the bank.
The final arrangement selected for this unit was an angled
Bank inlet
baffle (A in Figure 2), which spanned the width of the unit and
effectively acted as a set of ladder vanes. When flow encountered
the baffle from the left (upstream side), it was immediately
turned down toward the bank, which reduced much of the
rearward separation. The resulting FAE ratio was vastly improved,
peaking in the front section due to high approach flow near the
front of the bank. ■
-John F. Drennen, PE (jdrennen@asheroision.com) works for
Drennan Engineering Inc. in Windsor, Conn.
Division
wall
Flow
(front pass)
FAE area
3. Successfully controlling FAE. Relative FAE shows measured/calculated
gas and fly ash distributions compared to the ideal,
uniformly distributed flows. The baseline curve shows FAE distribution
without flow-modifying baffles, indicating two areas of high FAE. The
cascade baffle distribution shows an improvement in FAE near the rear
wall but increased FAE in the center of the bank. The angled baffle distribution
demonstrates what can be achieved with a properly designed
FAE baffle system. Source: Drennan Engineering Inc.
Baseline
20
18
16
14
12
10
8
6
4
2
0.00
Front wall
0.20
0.40
0.60
Front to rear distance (feet)
Cascade
Angled
Flow
(rear pass)
Maintaining Grid Reliability with a High
Renewables Portfolio
Many governments have adopted the goal of achieving 20% renewable
power generation by 2020. For California, the goal is to
reach 33% by 2020.
The state definitely has the sunshine needed, and in addition
to 2,000 MW of photovoltaic installations, there are the 354 MW
of Solar Energy Generating Systems that have been operating in
the Mojave Desert since the 1980s, soon to be joined by the 392MW
Ivanpah concentrating solar thermal plant currently under
construction.
The state also was the first to install utility-scale wind generation.
Although Texas and Iowa have since passed California
in terms of installed capacity, California added 921 MW in 2011,
bringing its total to 3,927 MW. According to the American Wind
Energy Association, another 18,269 MW of wind projects are in
the queue.
But alternative energy sources alone are not enough to provide
reliable generation or to stabilize the grid.
" One of the main issues that we are trying to raise aware0.80
1.00
Rear
wall
ash and gas separation toward the rear wall, with significant FAE
occurring in areas identified with hashes.
Figure 3 shows the relative FAE ratio at the top bank of the
rear pass for three different conditions. This ratio is based on
measured and computer model calculations for both ash and gas
distribution at the bank inlet as compared to uniformly distributed
flow across that plane. The baseline curve (original design
without baffles) shows significantly elevated FAE in the center
and near the rear wall due to flow separation. Both areas have
elevated ash and gas flows in the maximum FAE areas.
22
ness about is the need for flexible capacity on the system, " said
Stephanie McCorkle, spokesperson for the California Independent
System Operator (CAISO). " Gas-fired generation is our only means
right now until storage and demand response has matured to
compensate for the fluctuations of wind and solar power, so we
are actively educating policy makers and trying to propose a solution
at the regulatory level as well to ensure that there is still
enough conventional gas-fired generation available to us to be
able to maintain reliability as we see more and more renewables
added to the grid. "
This includes having enough reactive power to stabilize the
grid and allow the remotely generated renewable energy to reach
population centers. Both the generation and transmission issues
are easily addressable with existing technology.
A Problem of Predictability
The first problem with high renewable penetration is that wind
and solar are not dispatchable. Figure 4 from CAISO shows
a projected mix of fossil, wind, and solar energy generation
smoothly ramping up and down throughout the day to meet
changes in load. If that worked in the real world, there would
be no problem. Figure 5, however, shows how it works in the
real world. Although wind power may average out to a nice
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POWER | October 2012
Relative FAE rate
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POWER October 2012

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